# modeling/src/geometries/poly3/isConvex.js

``````const plane = require('../../maths/plane')
const vec3 = require('../../maths/vec3')

/**
* Check whether the given polygon is convex.
* @param {poly3} polygon - the polygon to interrogate
* @returns {Boolean} true if convex
* @alias module:modeling/geometries/poly3.isConvex
*/
const isConvex = (polygon) => areVerticesConvex(polygon.vertices)

const areVerticesConvex = (vertices) => {
const numvertices = vertices.length
if (numvertices > 2) {
// note: plane ~= normal point
const normal = plane.fromPoints(plane.create(), ...vertices)
let prevprevpos = vertices[numvertices - 2]
let prevpos = vertices[numvertices - 1]
for (let i = 0; i < numvertices; i++) {
const pos = vertices[i]
if (!isConvexPoint(prevprevpos, prevpos, pos, normal)) {
return false
}
prevprevpos = prevpos
prevpos = pos
}
}
return true
}

// calculate whether three points form a convex corner
//  prevpoint, point, nextpoint: the 3 coordinates (Vector3D instances)
//  normal: the normal vector of the plane
const isConvexPoint = (prevpoint, point, nextpoint, normal) => {
const crossproduct = vec3.cross(
vec3.create(),
vec3.subtract(vec3.create(), point, prevpoint),
vec3.subtract(vec3.create(), nextpoint, point)
)
const crossdotnormal = vec3.dot(crossproduct, normal)
return crossdotnormal >= 0
}

module.exports = isConvex
``````